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habitat, and biological environment as the primary constituent elements. This <br />includes a quantity of water of sufficient quality that is delivered to a <br />specific location in accordance with a hydrologic regime that is required far <br />the particular life stage for each species. Water depletions reduce the <br />ability of the river system to provide the required water quantity and <br />hydrologic regime necessary for recovery of the fishes. The physical habitat <br />includes areas of the Colorado River system that are inhabited or potentially <br />habitable for use in spawning and feeding, as a nursery, or serve as corridors <br />between these areas. In addition, oxbows, backwaters, and other areas in the <br />100-year floodplain, when inundated, provide access to spawning, nursery, <br />feeding, and rearing habitats. Water depletions reduce the ability of the <br />river to create and maintain these important habitats. Food supply, <br />predation, and competition are important elements of the biological <br />environment. Food supply is a function of nutrient supply and productivity, <br />which could be limited by reduction of high spring flows brought about by <br />water depletions. Predation and competition from nonnative fish species have <br />been identified as factors in the decline of the endangered fishes. Water <br />depletions contribute to alterations in flaw regimes that favor nonnative <br />fishes. The Service concludes that water depletions impact the primary <br />constituent elements; however, reasonable and prudent alternatives have been <br />developed by the Service that would avoid destruction or adverse modification <br />to the proposed critical habitat. <br />BIOLOGICAL BACKGROUND <br />Colorado Sauawfish <br />The Colorado squawfish evolved as the main predator in the Colorado River <br />system. The diet of Colorado squawfish longer than 3 or 4 inches consists <br />almost entirely of other fishes (Vanicek and Kramer 1969). The Colorado <br />squawfish is the largest cyprinid fish (minnow family) native to North America <br />and, during predevelopment times, may have grown as large as 6 feet in length <br />and weighed nearly 100 pounds (Behnke and Benson 1983). These large fish may <br />have been 25-50 years of age. <br />Based on early fish collection records, archaeological finds, and other <br />observations, the Colorado squawfish was once found throughout warmwater <br />reaches of the entire Colorado River Basin, including reaches of the upper <br />Colorado River and its major tributaries, the Green River and its major <br />tributaries, and the Gila River system in Arizona (Seethaler 1978). Colorado <br />squawfish were apparently never found in colder, headwater areas. Seethaler <br />(1978) indicates that the species was abundant in suitable habitat throughout <br />the entire Colorado River basin prior to the 1850's. Historically, Colorado <br />squawfish have been collected in the upper Colorado River as far upstream as <br />Parachute Creek, Colorado (Kidd 1977). <br />A marked decline.in..Colorado.squawfish popu.l.ati.ons can be closely correlated <br />with the construction of dams and reservoirs during the 1960's, introduction <br />of nonnative .fishes, a.nd removal of water from the Colorado River system. <br />Behnke and Benson (1983) summarized the decline of the natural ecosystem. <br />They pointed out that dams, impoundments, and water use practices are probably <br />the major reasons for drastically modified natural river flows .and channel <br />